Optimized stitching

This is an optimization of the mesh stitching function SerialMesh::stitching_helper(). Two calls to UnstructuredMesh::find_neighbors() are avoided by manually copying and updating the elements' lists of neighbors.

A new example (miscellaneous_ex10) is included to test this new feature. 8 cubes are stitched together in the domain [-1,1]^3 with both the old and optimized methods and a linear elasticity problem is solved using those meshes as well as an unstitched one. Solutions are compared in the L2 norm and profiling information is displayed.

Preliminary tests show over 60% stitching time reduction, with identical solutions.